The present invention relates to an illumination device using a light emitting element such as a light emitting diode (hereinafter referred to as “LED”).
In a conventional illumination device using a light emitting element such as an LED, countermeasures are taken to make it waterproof in consideration of the outdoor installation. At least one waterproofing technique has previously been employed in which a case is filled with a transparent resin.
For example, Japanese Unexamined Patent Publication No. 2005-302483 discloses an illumination device providing: a substrate with an LED mounted thereon; and a transparent and nearly box-shaped case with a back surface formed as an opening, wherein: the substrate is stored in the case from the opening; the substrate is supported with a space provided between the substrate and a front inner wall of the case; a transparent filling material is injected and filled in the case along a following path from the opening to the case; the filling material is cured to fix the substrate and the case integrally; and thus the LED along with the substrate are embedded in the filling material in the case and hermetically sealed, thereby ensuring a sufficient waterproof property. When the LED is lighted, the light is emitted through the transparent filling material and the case.
A white or blue-colored LED used for the purpose of the illumination typically has a structure in which an LED chip that emits blue light (light with wavelength of approximately 380 to 480 nm) is sealed with a resin containing a phosphor (e.g., silicon, epoxy). The conversion to a desired color of light is performed by means of the light with a wavelength region of approximately 480 to 780 nm that is converted by exciting the phosphor by the blue light emitted from the LED chip, and blue light that passes through the resin without being converted. Also, in the conversion to a desired color of light, the color of light is adjusted by, for example, changing a mixing ratio of the phosphor in the case where a top surface of the sealing resin containing the phosphor is an air layer. It is not premised on filling the top surface of the sealing resin containing the phosphor with the transparent resin (e.g., silicon).
When the case that stores the LED is filled with translucent silicon for the purpose of waterproofing, a refractive index of the sealing resin for the LED is almost equal to that of the silicon to be filled for the purpose of waterproofing. Therefore, there is a problem that the color temperature significantly shifts to a side of high color temperature due to a difference of the ratio between the light converted by exciting the phosphor by the blue light emitted from the LED chip, and the blue light that passes through without being converted, as compared to the case where the top surface of the sealing resin containing the phosphor is the air layer. An increase of the ratio of the blue light makes a white LED appear to be bluish white, and a bulb-colored LED appear to be yellowish white.